This invention relates to a laminated metal composite, comprising alternating layers of low flow stress material and high flow stress material, and formed using flow constraining elements around each low flow stress layer; and a method of making same.
A composite is a combination of at least two chemically distinct materials with a distinct interface separating the two materials. A metal matrix composite (MMC) is a composite material composed of a metal and a nonmetallic reinforcing agent such as silicon carbide (SIC) or graphite in continuous or discontinuous fiber, whisker, or discrete particulate form. A laminate is a material composed of several bonded layers. It is possible to have a laminate composed of multi-layers of a single type of material bonded to each other. However, such a laminate would not be considered to be a composite. The term "laminated metal composite" (LMC), as used herein, is intended to include a structural material composed of: (1) layers of metal or metal alloys interleaved with (2) a different metal, a metal alloy, or a metal matrix composite (MMC) containing strengthening agents.
It is possible to form a laminate containing two or three different materials with or without a bonding agent. Practical applications are discussed in E. S. Wright et at., "Laminated-Metal Composites", in Metallic Matrix Composites, ed. K. G. Kreider, 1974, Academic Press, New York, at pp 37-99. Examples of laminated materials include multi-layer plywood, multiple ply laminates of boron or carbon fiber-reinforced plastics, and Arall laminates (Aramid fiber reinforced aluminum alloy laminates). Most of the industrial metal laminates, however, contain only two or three layers with thin surface layers cladding a thicker substrate material. Such cladded metal laminates have been used for several decades, due to their cost effectiveness and resistance to wear and corrosion. Kum et at., in J. Mech. Phys. Solids, Vol. 31, 1983, at pp. 173-186, describes metal laminates made of two component metals such as art ultrahigh carbon steel and mild steel. Chowla et at. in Proc. 2nd International Conference on Composite Materials, 1978, discuss, at pp. 1237-1245, the forming of metal laminates composed of aluminum alloy and stainless steel using press or roll bonding techniques. However, metal laminates having five or more component layers are virtually non-existent.
Kum et at., in J. Mech. Phys. Solids, Vol. 31, 1983, at pp. 173-186 have shown that multi-layer laminated metal composites (LMCs) can have damage critical properties superior to those of their component materials. Damage-critical properties essentially represent the resistance to crack initiation and propagation. The resistance to crack propagation is improved in LMCs, because the propagating crack is blunted or eliminated by interlayer delamination. Crack initiation by fatigue and impact can be suppressed by applying a crack-resistant hard layer on the surface.
Laminated metal composites (LMCs) formed from alternate layers of high strength and low strength materials have been found to have excellent damping, fatigue, fracture, wear, and impact properties and could be useful as structural materials for machine tools, buildings, transportation vehicles, and aircraft.
The stress of a component layer impacts the quality of a laminate upon deformation, such as compression rolling. Stress is defined as the load per unit of area. The stress of an individual component layer can be referred to as the flow stress of the layer. In addition, the flow stress may be regarded as the quantity of stress which will cause the material to flow at a given strain and predetermined temperature. A layer with low flow stress flows much more readily than a layer having high flow stress. The flow stress of a material is thus understood to be a relative term. Generally, soft metals have a lower flow stress that hard metals.
Laminates containing two or three layers are made by roll-bonding. Multilayer laminates have been made by stacking alternate layers of component materials and then press-bonding or roll-bonding the resulting stack at an elevated temperature by imposing a large deformation, f the component materials are not too different.
However, laminated metal composites (LMCs) containing component layers having substantially dissimilar flow stresses, i.e., differing by greater than at least 10%, and usually differing by an order of magnitude or more, are difficult to produce by press bonding since those component layers having the lower flow stress will deform prematurely and extrude out of the stack. For example, laminates of a high strength steel and a low strength aluminum alloy are difficult, if not impossible to make by press bonding a simple stack of such differing materials at an elevated temperature near the melting point of the aluminum alloy. In the latter case, the flow stress of the aluminum alloy is far smaller than that of the steel component at the same temperature.
Referring to prior art FIGS. 1 and 2, an initial stack of such dissimilar materials is illustrated at 2 in FIG. 1. Layers 4, 8, and 12 comprise a high flow stress material such as a high strength steel, while intermediate layers 6 and 10 comprise a low flow stress material such as a low strength aluminum alloy. When pressure is applied to this stack, such as by the aforesaid compression roll bonding, much of the low flow stress layers 6 and 10 extrude from the stack, as shown at 16 and 18 in prior art FIG. 2, resulting in the formation of very thin low flow stress layers 6' and 10', as also shown in FIG. 2.
Nakatate, in Japan Patent document 60-148688, addressed this problem of bonding a stack of plate materials having a large difference in deformation resistance by providing frame members around each side of a rectangular stack, then welding the frame members to the stack, and then passing the welded stack through press welding rolls to bond the layers of the stack to one another. While this method apparently provides the desired retention of the low flow stress material in the stack, it requires the provision of weldable materials in the stack to weld the stack to the individual frame members placed on each side of the illustrated rectangular stack.